The subject of the present invention is a drum used for the assembly of tires. More precisely, the invention relates to a drum used for shaping during assembly.
One known technique for assembling a radial carcass tire comprises generally what is called a building stage followed by a shaping stage. During the first stage, generally carried out on a first drum, a first layer of air-impermeable rubber is typically laid on a cylindrical assembly drum having a horizontal axis. Then the carcass ply is laid, the reinforcement cords of which are substantially parallel to the axis of the drum. Then the two circular beads are built, each usually composed of a bead wire and a filler. Finally, the two edges of the carcass ply are turned up around the beads and the sidewalls are laid.
Having thus obtained a cylindrical carcass, it is necessary to pass to the shaping stage to impart to the carcass a roughly toroidal shape before laying the tread on the periphery thereof. Most frequently, this carcass is transferred onto another drum, referred to as a second-stage or shaping drum, comprising two bead holding systems intended to receive the beads. Then the two bead holding systems are brought together axially simultaneously while shaping the tire by inflation.
Finally, a crown, or cylindrical reinforcement, and the tread, are laid on the carcass thus shaped. The raw tire is then ready for curing in a mold, in which vulcanization will impart the final shape thereto. A shaping drum without a membrane is described in U.S. Pat. No. 3,402,090 (see in particular FIG. 2 thereof) which is formed of two bead holding systems which are radially expandable to be able to be applied in tight manner to the interior of the beads, before being brought together to shape the carcass by inflation. U.S. Pat. No. 4,325,764 describes another device of the same type, in which the bead holding systems of the beads comprise an internal axial circular stop. Once the carcass has been laid on the two bead holding systems, one begins by moving the bead holding systems apart from each other in order to bring the carcass into abutment, which results in it being properly centered. Then the bead holding systems are radially expanded and brought together, as above.
Shaping of a tire carcass is performed in the same way with the drum described in patent application JP 59 007 034, in which the two bead holding systems are made of radially expandable segments partially covered by an elastic membrane.
However, it has for a long time been noted that the shaping operation is still a critical phase. In fact, for certain bead structures, it requires a rotational movement of the beads on themselves, accompanied by friction on said bead holding systems. This subjects the beads to very great stresses. Since at this stage of manufacture the various components of the beads are still in the raw state, they undergo deformations which are sometimes detrimental to the final quality of the tire.
Various solutions for facilitating this rotation of the beads have therefore been proposed. One of these consists in equipping the bead holding systems, which generally comprise a rigid circular support, with membranes which can be deformed or displaced at the time of shaping the carcass, accompanying the rotation of the beads to some extent. U.S. Pat. No. 5,246,515 relates to the assembly of tires for aircraft, which have particularly voluminous beads.
The device described comprises bead holding systems formed of radially expandable, fluted segments, covered by an elastic membrane which surrounds a circular return filler. At the time of rotation of the beads, the elastic membrane, clamped between the latter and the associated bead holding systems, becomes taut, deforming the filler, and accompanies the movement of the bead. However, this system has one disadvantage: the friction of the elastic membrane on the surface which supports it, during the rotation of the beads. Furthermore, the patent provides for lubrication of this zone in order to reduce the friction therein.